This invention relates generally to a centrifugal pump and, more particularly, to a centrifugal pump of the type having a variable output flow capacity and such as might be used as a fuel pump in an aircraft.
In supplying fuel to the engine of an aircraft, it is desirable to keep the temperature rise in the fuel passing through a fuel pump to a minimum for both high flow and low flow fuel requirements. As is disclosed in U.S. Pat. No. 3,784,318, issued to Donald Y. Davis, one way of accomplishing this in a centrifugal pump having diffuser vanes is to provide a shutter valve for closing off selected ones of the diffuser passages defined by the vanes. It will be appreciated, however, that with this type of pump operating at a low flow discharge, an undesirable pulsing flow of fluid may be introduced into the pumping system downstream of the impeller because of the closing off of several of the diffuser passages. Moreover, the precise machining required for the vanes and the complicated assembly of the vanes in the pump housing makes this type of pump quite expensive to produce.
Another centrifugal pump including a shutter valve for producing a variable flow discharge is disclosed in U.S. Pat. No. 3,826,586. The pump disclosed in this patent apparently utilizes a volute type diffuser instead of vane diffusers. The variable flow is produced by restricting the flow from the impeller tip into the volute surrounding the impeller through the use of a shutter valve of similar construction to the valve disclosed in the Davis patent. By restricting the output flow of the fluid from the impeller into the volute in this manner, however, turbulence may be introduced in the fluid, causing losses in the conversion of velocity head into pressure head. As a result, at lower flowrates, the discharge pressure may not reach the magnitudes desired for proper operation of the aircraft engine, and the efficiency of the pump may be impaired substantially.